The Trump administration said Thursday night that it will not defend the Affordable Care Act against the latest legal challenge to its constitutionality — a dramatic break from the executive branch’s tradition of arguing to uphold existing statutes and a land mine for health insurance changes the ACA brought about.

In a brief filed in a Texas federal court and an accompanying letter to the House and Senate leaders of both parties, the Justice Department agrees in large part with the 20 Republican-led states that brought the suit. They contend that the ACA provision requiring most Americans to carry health insurance soon will no longer be constitutional and that, as a result, consumer insurance protections under the law will not be valid, either.

The three-page letter from Attorney General Jeff Sessions begins by saying that Justice adopted its position “with the approval of the President of the United States.” The letter acknowledges that the decision not to defend an existing law deviates from history but contends that it is not unprecedented.

The bold swipe at the ACA, a Republican whipping post since its 2010 passage, does not immediately affect any of its provisions. But it puts the law on far more wobbly legal footing in the case, which is being heard by a GOP-appointed judge who has in other recent cases ruled against more minor aspects.

The administration does not go as far as the Texas attorney general and his counterparts. In their suit, lodged in February in the U.S. District Court for the Northern District of Texas, they argue that the entire law is now invalid.

By contrast, the Justice brief and letter say many other aspects of the law can survive because they can be considered legally distinct from the insurance mandate and such consumer protections as a ban on charging more or refusing coverage to people with preexisting medical conditions.

A group of 17 Democratic-led states that have won standing in the case also filed a brief on Thursday night arguing for the ACA’s preservation.

While the case has to play out from here, the administration’s striking position raises the possibility that major parts of the law could be struck down — a year after the Republican Congress failed at attempts to repeal core provisions.

In an unusual filing just before 6 p.m. Thursday, when the brief was due, the three career Justice attorneys involved in the case — Joel McElvain, Eric Beckenhauer and Rebecca Kopplin — withdrew.

The department’s argument, if adopted by U.S. District Judge Reed O’Connor, “would be breathtaking in its effect,’ said Timothy Jost, a retired Washington and Lee law professor who follows such litigation closely. “Of all of the actions the Trump administration has taken to undermine individual insurance markets, this may be the most destabilizing. . . . [If] I’m an insurer, I don’t know what I am supposed to do or not.”

Jost, an ACA supporter, noted that the administration’s decision not to defend the law comes during the season when participating insurers must file their rates for next year with state regulators. It raises new questions about whether insurers still will be required to charge the same prices to all customers, healthy or sick.

And Topher Spiro, vice president of health policy at the liberal Center for American Progress, said the administration’s legal argument contradicts promises by Trump that he would not tamper with the ACA’s protections for people with preexisting medical conditions.

University of Michigan law professor Nicholas Bagley, another ACA defender, went even further in a blog post. “If the Justice Department can just throw in the towel whenever a law is challenged in court, it can effectively pick and choose which laws should remain on the books,” he wrote. “That’s not a rule of law I recognize. That’s a rule by whim. And it scares me.”

Crusading against the ACA has been a priority of Trump’s since his campaign for the White House. On his first night in office, Trump issued an executive order, directing federal agencies to lighten the regulatory burden placed by the law. Last October, the president unilaterally ended a significant part of the law that cushions insurers financially from an obligation to give discounts to decrease out-of-pocket costs to lower-income customers with ACA coverage.

More recently, the White House and Department of Health and Human Services have been working to make it easier for consumers to buy relatively inexpensive health plans that exclude some of the benefits the ACA requires.

The new challenge comes six years after the Supreme Court’s divided ruling that the ACA is constitutional. That ruling hinged on the reasoning that, while the government “does not have the power to order people to buy health insurance,” as Chief Justice John G. Roberts Jr. wrote for the majority, it “does have the power to impose a tax on those without health insurance.”

The case in Texas, which has attracted relatively little notice until now, emerges from the massive tax bill Congress passed late last year. In that, lawmakers decided to eliminate the tax penalty the ACA requires people to pay if they flout the insurance mandate. The enforcement of that requirement will end in January.

As a result, the Texas lawsuit contends, “the country is left with an individual mandate to buy health insurance that lacks any constitutional basis. . . . Once the heart of the ACA — the individual mandate — is declared unconstitutional, the remainder of the ACA must also fall.”

Texas and the accompanying states have asked for a preliminary injunction that could suspend the entire law while the case plays out in court.

But the administration disagrees with that position. Instead, Justice officials argue in their brief that the ACA’s insurance requirement will not become unconstitutional until January, so that “the injury imposed by the individual mandate is not sufficiently imminent” and that the judge could issue a final ruling in the case before then.

O’Connor, who is hearing the suit, was appointed by President George W. Bush and has ruled against the ACA in other cases the past few years.

Until Thursday’s filing, the Trump administration had not indicated its position on either this latest lawsuit or the Republican states’ effort to block the law while the case moved along.

Amy GoldsteinAmy Goldstein is The Washington Post’s national health-care policy writer. During her 30 years at The Post, her stories have taken her from homeless shelters to Air Force One, often focused on the intersection of politics and public policy. She is the author of the book "Janesville: An American Story." Follow

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